Tektronix publish a nice short guide - 'Fundamentals of Floating Measurements and Isolated Input Oscilloscopes'. (copy/paste that into google...)

NB Tek DO NOT recommend using a mains isolation transformer to 'float' a scope, ever.

Meanwhile that's an elegant technique, Elvee. I've only used small toroids but found with the sig gen that with a nominal loading - say 1K across the secondary - regular smallish mains step-down toroids can be quite good enough to present a flat passband beyond at least ~30Khz to a soundcard. More than enough for what I was interested in, and cheap to try.

Edit to add - last toroids I measured had under 500pF stray C pri:sec (650VA 35-0-35VAC in my power amps, actually). That's the real reason for a nominal secondary load on top of any two-resistor potential divider to set your signal input level.

@sim0n #2:
Yes, probe's ground is connected to chassis GND.
I think I can't use the scope to measure the mains voltage directly because the RCD will trip due to the internal connection chassis GND to mains earth. TT or TN system, must look at it.
My intention was to use a 240/240 isolation transformer to power the oscilloscope so that it 'floats', thus permitting me to connect scope's chassis GND to Neutral and the probe to Live. Of course EXTREME care is mandatory when dealing with mains voltages.

The same isolation transformer could be used to power the desktop PC. Then a 100:1 resistive attenuator will bring the 240VAC within soundcard's range. I feel that a laptop running off its batteries could also be used and no transformer is required.

sim0n and DF96: I think the transformer will filter out some of the components that corrupt the mains power. My intention is to expose and record the contamination of mains power.

@DF96 #3: very clever the idea of a resistor to load the secondary in order to avoid saturation. I had forgotten that, thanks.

@AndrewT #4: Yes, to investigate the components of mains power.

@Elvee #9: indeed, transformers can do a lot of things. I had read somewhere that transformers can be used as power conditioners, up to a point ofc.

Also, if you're going to be monitoring this all day do you need the exact trace of each individual disturbance or are you just interested in the frequency of how often they occur (i.e. every event is bad anyway)

Right, the input impedance of the scope is >1MOhm. It's doubtful whether the oscilloscope allows the user to disconnect the mains earth from the probe GND. I will look at it anyway. Perhaps some other scope has a switch that connects/disconnects the mains earth to chassis.

The soundcard will do the monitoring and the recording, I will use the oscilloscope occasionally to take pictures of the disturbances. (if I am around and one occurs)

I would use a battery powered, floating, 3000V Opto Isolated "sensor head" to sample line voltage/waveform/whatever and then use standard stuff on "this side" of the barrier.
Which by the way is the way Pros do it.
I'd never never never ever connect any scope probe to any live wire.
Not even neutral !!!
Ground is fine , of course.

Yes, that's a practical approach.
You could use any regular, say, 220:12V transformer, resistively load its secondary to put magnetizing current and low level hysteresis out of the way (or to be more precise, to make conditions more realistic) and measure that.
It will be an accurate sample of what happens at the primary side.

.
You could use any regular, say, 220:12V transformer, resistively load its secondary to put magnetizing current and low level hysteresis out of the way (or to be more precise, to make conditions more realistic)

Resistive loading won't change anything to magnetizing current or hysteresis.
It could slightly alter the magnetizing current if it is unduly heavy, but anyway the magnetizing current per se is not a problem, the saturation is.
On the other hand, loading will significantly degrade the reproduction of harmonics. It will also slightly magnify the effect of hysteresis.